allow specifying a required version in GUILE_PROGS

* meta/guile.m4: GUILE_PROGS now takes an optional argument to specify a
  required Guile version. By default, it requires Guile >= 2.0. A micro
  version can also be specified (e.g. GUILE_PROGS([2.0.10])).

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Thank Aleix.